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I know of three instances of chains breaking on Ducati belt drive engines.
One was a Pantah and two were Pasos.
The result of each of these failures was a smashed crankcase near the drive sprocket, in every case caused by the plastic protector for the swing arm becoming dislodged and going straight between the chain and sprocket teeth. There is not enough room to accommodate the chain as it is lifted off the sprocket, which is what happens. The chain breaks through to the inside of the crankcase, near the gear selector mechanism. In doing so, the chain was broken and tried to wrap around the sprocket, cutting grooves into the swingarm pivot, which is also close to the sprocket.
The protectors are secured by one bolt/nut. They evidently can become distressed in use, to the point that they can break away from the single attachment point.
The new replacement protector for the '89 906 I'm working on is secured in the usual way, but in addition a polyurethane adhesive (tough stuff) was applied between the flat bottom surface and the top of the swingarm, and should help prevent problems in future.
Perhaps more recent models don't have this weakness?

Thanks for the tip. I'll keep an eye on mine.

This was a contentious issue back when the Pantah engines were raced. There were some after-market chain guards available. I made one for my 906 that replaced the plastic sprocket cover. It bolted on to the existing housing but had a steel band welded to the inside of the cover that ran along the outside of the chain run. Idea being that if the chain went it never reached the crankcase but was dumped out under the swingarm.

The fact that you mention the swing arm being chewed by the chain is a clue ,a chain that is to loose will flick up off the sprocket, chew right thru the chain slider and hit the swingarm , and the only way a chain can wind onto the sprocket is if its already broken .....lack of proper maintainance or just plain worn out. The chain dont break because the slipper comes adrift ( more lack of maintainance ) . A whole slipper will not go round the sprocket , however if it has already been chewed in half by a loose flailing chain the top half might , but we are back to the lack of maintainance ....again. There is no design flaw , you just need to maintain your bike . I get sick of listening to people who literally "drive it till it drops " and then come up with a bunch of unplausable theories why its not their fault. The above mentioned home made ( and supplied aftermarket for some bikes , usually moto x or similar ) chain guide will stop a broken chain from bunching up on top of the sprocket but if the broken end continues round the sprocket and starts to wind around the sprocket even that wont save your cases. The Paso takes a 520 chain and you need to get a good quality heavy duty chain made for a 900cc bike ....not a 250cc mx /trail bike chain ( with skinny side plates ) .....which a lot of peeps do coz its cheaper.

I'm well aware of the possibility of a broken chain bunching up at the gearbox sprocket and smashing the cases and have seen a few examples, none of them mine fortunately. I have case savers fitted to all my bikes to help prevent this. What is news to me is that the plastic guide over the swinging arm can come adrift and jam in the sprocket, breaking the chain. Although it's not surprising when you think about it.

I did fit case savers ( forgot what they were called ...thanks )to my trail bikes, and if I couldnt buy one I made it . I think its fair to say that most modern ish bikes are very similar in the sprocket and case area and the potential for disaster is pretty much the same. Im just waiting for my brother in laws T100 (new 790cc model )Triumph to do this (a total #uckwitt) as he seems to think the specified 25mm chain chain tension should be measured from the ground, and the bike probly hasnt had a lick of maintainance since I give up trying to help him 5 years ago. Shame , it was a nice bike when his Mum bought and paid it off for him.

Mc tool wrote:The fact that you mention the swing arm being chewed by the chain is a clue ,a chain that is to loose will flick up off the sprocket, chew right thru the chain slider and hit the swingarm , and the only way a chain can wind onto the sprocket is if its already broken .....lack of proper maintainance or just plain worn out. The chain dont break because the slipper comes adrift ( more lack of maintainance ) . A whole slipper will not go round the sprocket , however if it has already been chewed in half by a loose flailing chain the top half might , but we are back to the lack of maintainance ....again. There is no design flaw , you just need to maintain your bike . I get sick of listening to people who literally "drive it till it drops " and then come up with a bunch of unplausable theories why its not their fault.

That is blaming the victim.
There was no chain damage to the swing arm, except at the very front edge of it, where the chain had bunched up between the sprocket and swing arm.
Only the mounting ear of the plastic chain slider was still attached to its mounting point on the swing arm.
The rest of it was mangled around the front sprocket with the chain.
The damage to the crankcase would have happened even if the chain hadn't broken, as the slider forced the chain out where there was not enough space for it.